FIG. 9 is a schematic diagram illustrating a latent heat recovery type-gas water heater provided with a sensible heat recovery type-primary heat exchanger (311) and a latent heat recovery type-secondary heat exchanger (321) disposed in this order below a combustion surface (30a) of an all primary air combustion type burner (3a). In the water heater, all the air taken in through an air supply passage (43) by rotating a fan (not shown) in a fan casing (41) is fed as primary combustion air to a mixing device (42) and the air is mixed with fuel gas supplied through a gas supply passage (44) in the mixing device (42). Subsequently, a mixture gas obtained by mixing is fed to a reverse-combustion type burner (3a) in such a manner that the combustion surface (30a) is disposed facedown.
In the water heater describe above, since the combustion surface (30a) of the burner (3a) is disposed facedown, during a hot-water supply operation, combustion exhaust gas is ejected downward together with flames of the burner (3a) by airflow of the fan. The primary heat exchanger (311) recovers sensible heat when the combustion exhaust gas is fed to the primary heat exchanger (311), and the secondary heat exchanger (321) recovers latent heat when the combustion exhaust gas is fed to the secondary heat exchanger (321). Then, the combustion exhaust gas is discharged outside the water heater from an exhaust port (351) through an exhaust duct (341). Further, part of the combustion exhaust gas recovered the latent heat by the secondary heat exchanger (321) condenses into acid drain to remain in the secondary heat exchanger (321).
In the water heater described above, when combustion of the burner (3a) is turned off by a stop of the hot-water supply operation, the air flow of the fan stops. In this condition, since the burner (3a) is disposed above the heat exchangers (311) and (321), the acid drain remaining in the latent heat recovery type-secondary heat exchanger (321) disposed below the burner (3a) evaporates to generate acid vapor, which ascends in a vessel (40). Thus, the acid vapor may flow beyond the burner (3a) and into the fan casing (41) and the mixing device (42). As a result, if the acid vapor condenses in those members, the blades of the fan rust in the fan casing (41) and the mixing device (42) corrodes.
It is considered to dispose a check valve (4) as a backflow preventing member between the vessel (40) provided with the burner (3a) and the fan casing (41) accommodating the fan in such a manner that the acid vapor does not reach upstream of the burner (3a) when the fan stops.
When the fan rotates, the mixture gas obtained by mixing the air and the fuel gas in the mixing device (42) is fed to the vessel (40) provided with the burner (3a) through the check valve (4). On the other hand, by disposing the check valve (4) between the vessel (40) and the fan casing (41), when the fan stops, the acid vapor generated in and ascending from the vessel (40) is blocked by the check valve (4), whereby the flow of the acid vapor into the upstream fan casing (41) and mixing device (42) can be prevented.
For example, as shown in FIG. 10, the check valve (4) is disposed between an upstream connecting pipe (40a) of the vessel (40) and a downstream connecting pipe (41a) of the fan casing (41) connecting thereto for convenience in maintenance. Preferably, the check valve (4) is provided in the vicinity of an open end of either the upstream connecting pipe (40a) or the downstream connecting pipe (41a).
FIG. 10 is a partial enlarged cross-sectional view showing a connection state of passages. As shown in FIG. 10, when the check valve (4) is accommodated in the upstream connecting pipe (40a) of the vessel (40), an annular sealing packing (45) is disposed between opposite connection end surfaces of the connecting pipes (40a) and (41a) to prevent leakage from a connected portion of the upstream connecting pipe (40a) of the vessel (40) and the downstream connecting pipe (41a) of the fan casing (41).
Moreover, if the leakage from a gap between an outer surface of the check valve (4) and an inner surface of the upstream connecting pipe (40a) of the vessel (40) occurs, function as the check valve may be impaired. In order to prevent the leakage from the inner surface, an O-ring (46) is fitted onto the check valve (4). Not only the vessel (40) and the fan casing (41) are connected in airtight state, but the leakage from the gap between the inner surface of the upstream connecting pipe (40a) of the vessel (40) and the outer surface of the check valve (4) is prevented, by these two sealing members of the sealing packing (45) and the O-ring (46).
On the other hand, it is necessary for the all primary air combustion type burner (3a) to supply the mixture gas of the air and the fuel gas into the vessel (40) by the fan. Thus, when the all primary air combustion type burner (3a) is used, load on the fan is higher than that of a Bunsen burner. Therefore, it is desirable that flow resistance in the passage from the fan to the burner (3a) in the vessel (40) via the downstream connecting pipe (41a) of the fan casing (41) and the upstream connecting pipe (40a) of the vessel (40) is as low as possible.
However, in a case where the check valve (4) onto which the O-ring (46) is fitted is disposed in the upstream connecting pipe (40a) of the vessel (40) as described above, the passage will be narrowed and the flow resistance will be increased. In addition, the water heater described above requires the two sealing members of the sealing packing (45) and the O-ring (46) to keep airtightness between the upstream connecting pipe (40a) of the vessel (40) and the downstream connecting pipe (41a) of the fan casing (41). Accordingly, a number of components increases, which results in complicating an assemble work.